Annex D of the “ATSC Digital Television Standard” was published by the Advanced Television Systems Committee (ATSC) in September 1995 as its document A/53. This standard defined the broadcasting of digital television (DTV) signals within the United States of America and is referred to in this specification simply as “A/53”. A/53 specifies the transmission of DTV signals in MPEG-2-compliant data packets the bits of which are randomized by being exclusive-ORed with bits of a prescribed pseudo-random binary sequence before being subjected to (207, 187) Reed-Solomon forward error correction coding. A/53 specifies that the resulting (207, 187) R-S FEC codewords are then subjected to convolutional byte interleaving that spreads their bytes to occur at 52 byte intervals. A/53 specifies that the resulting byte-interleaved codestream is then subjected to 12-phase ⅔ trellis coding. A/53 specifies that the resulting symbols are mapped to a vestigial-sideband amplitude-modulation signal in which the digital symbols are transmitted by 8-level modulation. This 8-level modulation, known as 8VSB, has +7, +5, +3, +1, −1, −3, −5 and −7 normalized modulation signal values.
The transmission of more robust DTV signals became a subject of interest at the beginning of the twenty-first century. Any robust DTV signal transmission had to be of such nature as to permit time-division interleaving with ordinary 8VSB signal transmissions, so as not completely to obsolete legacy DTV receivers. Accordingly, any robust DTV signal transmission had to be susceptible to convolutional byte interleaving and subsequent 12-phase ⅔ trellis coding together with any accompanying ordinary 8VSB signal transmission.
One approach to improving the robustness of DTV transmissions by reducing code rate is to increase the amount of forward-error-correction coding of the digital data. An approach which introduces further Reed-Solomon coding and further trellis coding of the less significant bits of each symbol is described in a “ATSC Digital Television Standard, Revision C” published by the Advanced Television Systems Committee (ATSC) in July 2004. This revised standard is referred to as ATSC document A/53C with Amendment No. 1. This revised standard describes code rate being further reduced by applying trellis coding to the most significant bit of each symbol. An alternative approach to improving the robustness of DTV transmissions is to restrict the symbol alphabet to increase the distance between the levels of amplitude modulation used to form the symbols. Still another approach is to provide further inner coding following the outer (207, 187) Reed-Solomon forward-error-correction coding prescribed by A/53.
U.S. patent application Ser. No. 10/955,212 filed 30 Sep. 2004 by A. L. R. Limberg and titled “TIME-DEPENDENT TRELLIS CODING FOR MORE ROBUST DIGITAL TELEVISION SIGNALS” was published Apr. 7, 2005 with publication No. 2005-0074074. That application describes a previously known first type of robust modulation called “pseudo-2VSB”, or “P2VSB” and a previously known second type of robust modulation called “enhanced 4VSB”, or “E4VSB”. In P2VSB the digital symbols are restricted to +7, +5, −5 and −7 normalized modulation signal values, but sustain trellis coding. In E4VSB the digital symbols are restricted to +7, +1, −3 and −5 normalized modulation signal values, but sustain trellis coding. U.S. patent application Ser. No. 10/955,212 discloses a third type of modulation in which the symbol alphabet of a digital television signal is restricted in either of two alternative ways. In accordance with a prescribed pattern, a ZERO or a ONE is inserted after each bit in a data segment to be incorporated into a data field for randomization, R-S FEC coding, convolutional interleaving, and trellis coding. Inserting a ONE after each bit in a stream of randomized data causes the trellis coding procedure to generate a restricted-alphabet signal which excludes the −7, −5, +1 and +3 symbol values of the full 8VSB alphabet. Inserting a ZERO after each bit in a stream of randomized data causes the trellis coding procedure to generate a restricted-alphabet signal which excludes the −3, −1, +5 and +7 symbol values of the full 8VSB alphabet. This third type of modulation has been called “prescribed-coset-pattern modulation”, or “PCPM”. Each of these three types of robust modulation that restrict the symbol alphabet halves the code rate of ordinary 8VSB.
U.S. patent application Ser. No. 11/119,662 filed 2 May 2005 by A. L. R. Limberg and titled “DIGITAL TELEVISION SIGNALS USING LINEAR BLOCK CODING” teaches that halving the code rate again to achieve still more robust “super-robust” signal transmission by further restricting the symbol alphabet is infeasible. This is because the pattern of trellis coding A/53 prescribed for the less significant bits of 8VSB symbols has to be preserved within the data segments of each field of convolutionally interleaved data. Otherwise, legacy DTV receivers designed to receive 8VSB transmitted as prescribed by A/53 will not be able to receive 8VSB data segments successfully if those segments have been convolutionally interleaved with segments of robust data. So, further reduction of the code rate has to be done by additional coding that extends over a plurality of 8VSB symbol epochs.
U.S. patent application Ser. No. 10/955,212 further teaches that this additional coding should be such that it does not involve data transmitted at normal 8VSB code rate, nor robust data transmitted at one-half 8VSB code rate, which data are apt to be convolutionally interleaved with super-robust data transmitted at one-quarter 8VSB code rate or so. A binary linear block code can provide for such additional coding. To facilitate time-division multiplexing with data segments of ordinary 8VSB and data segments of restricted-alphabet symbols, it would be preferable that an integral number of blocks of the additional coding fall within an interval equal to a multiple of 828 symbol epochs of 8VSB. A (23, 12) binary Golay code and a (24, 12) extended binary Golay code are specifically considered in application Ser. No. 10/955,212. Shorter-length linear block codes of (16, 8), (15, 8), and (8, 4) types are also specifically considered. The (16, 8) and (8, 4) linear block codes are well-suited to providing an inner code for locating errors for an outer (207, 187) Reed-Solomon forward-error-correction code.
Early proposals for more robust DTV signals retained a three-byte header and twenty parity-check bytes of (207, 187) Reed-Solomon forward-error-correction coding in data segments containing reduced-code-rate information. The halving or quartering of code rate was confined just to the 184-byte portions of the 207-byte data segments in those early proposals. Accordingly, the robust transmission of an MPEG-2 data packet could not be completed within just two data segments, but required somewhat more than two data segments, complicating time-division multiplexing of the robust transmissions with transmissions of other code rate(s). Also, an even more robust “super-robust” transmission of an MPEG-2 data packet could not be completed within just four data segments, but required somewhat more than four data segments, complicating the time-division multiplexing of these more robust transmissions with transmissions of other code rate(s).
In 2006 engineers of Samsung Electronics Company, Ltd. proposed that robust DTV signals use turbo coding. More particularly, Samsung engineers proposed turbo coding that subsumed ⅔ trellis coding, to achieve a code rate nominally one-half that of ordinary 8VSB as prescribed by A/53. The Samsung engineers also proposed other turbo coding that further reduced code rate to nominally one-quarter that of ordinary 8VSB. These two types of turbo coding used by the Samsung engineers are referred to as “halved-code-rate A-VSB turbo coding” and “quartered-code-rate A-VSB turbo coding”, respectively. The Samsung engineers designed their turbo coding to be disposed within adaptation fields of the MPEG-2-compliant data packets subsequently transmitted according to the methods used for transmitting ordinary 8VSB. I.e., the turbo coding took the form of a so-called “private” data transmission.
Turbo coding appears to be gaining favor over other forms of redundant coding for use in robust DTV transmissions. In part this is because turbo coding can achieve performance under reception conditions for all-white Gaussian noise (AWGN) that approaches the Shannon limit more closely than many other forms of redundant coding. Another factor in its favor is that receiver complexity, while appreciable, is less than for competing types of coding that achieve similar or only slightly better performance under AWGN reception conditions.
Recapitulating, a general feature of many of the forms of robust DTV data that have been proposed is that each of them redundantly codes randomized MPEG-2-compliant data packets to reduce their code rate to 1/N that of ordinary 8VSB signal, N being a small whole number such as 2, 3, 4 or 5. In some proposals 184-byte chunks of the robust DTV data are included as payloads of MPEG-2-compliant data packets within (207, 187) R-S FEC codewords. In other proposals, such as that of Samsung Electronics Co., Ltd., smaller chunks of the robust DTV data are inserted into application fields of MPEG-2-compliant data packets within (207, 187) R-S FEC codewords.
U.S. patent application Ser. No. 10/955,212 teaches that the halving or quartering of code rate need not be confined just to the 184-byte portions of 207-byte data segments, but usefully extends over all 207 bytes of data segments. This is part of a more general concept of the inventor that chunks of robust DTV data preferably should completely fill 207-byte segments, which then are time-division multiplexed with 207-byte segments of ordinary 8VSB signal of the sort prescribed by A/53 as originally published in 1995. U.S. patent application Ser. No. 10/955,212 describes the following preferred succession of 207-byte segments of robust data in the data field offered for convolutional byte interleaving. The 207-byte segment transmitting one of the two halves of a (207, 187) R-S FEC codeword redundantly coded to a halved code rate is immediately succeeded by the 207-byte segment transmitting the other of those two halves. The successive four 207-byte segments transmitting quarters of a (207, 187) R-S FEC codeword redundantly coded to a quartered code rate are contiguous in the data field(s) offered for convolutional byte interleaving. Such arrangements of the portions of a (207, 187) R-S FEC codewords redundantly coded to a reduced code rate do not result in those codewords having substantially improved capability of withstanding sustained burst error. Indeed, the capability of withstanding sustained burst error may in fact be compromised.
Grouping the portions of redundantly coded (207, 187) R-S FEC codewords together in a prescribed way does simplify decoding the redundant coding. It also simplifies the task of describing the locations of redundantly coded (207, 187) R-S FEC codewords within data fields.